Mesoscale Precipitation Discussion 1026
NWS Weather Prediction Center College Park MD
936 PM EST Wed Nov 28 2018

Areas affected...Northern California

Concerning...Heavy rainfall...Flash flooding possible

Valid 290235Z - 291300Z

Summary...Heavy rain may affect portions of Northern Cailfornia
tonight, with rain rates over 0.5 in/hr in some areas. Some of the
rain bands could be convective in nature, with over 0.5 inch of
rain falling in as little as 20-30 minutes, which could produce
flash flooding in and near recent burn scar areas.

Discussion...GOES West water vapor imagery shows a strong +PV
anomaly near 40N/131W, and this is projected by model consensus to
shift east-southeast toward the northern California coast tonight.
12hr height falls become maximized around 12-15Z, suggesting that
the strongest forcing will arrive later in the night. This
corresponds with an increase in most-unstable LI, largely after
06Z, with values of -1 to -3C spreading well inland into the
Sacramento Valley. Therefore, after 06Z is when the primary wave
of rain should arrive, and based on the 18Z HREF, much of the
northern coastal ranges, Napa Valley, and Sacramento Valley region
should be affected. With moderate amounts of instability
available, convective banding seems to be a reasonable
expectation, and thus rain rates could be actually more focused
into short time periods than traditional model QPFs would suggest.

Certainly, rain rates of 0.5 in/hr appear likely to occur in some
portions of the region, and this could lead to flash flooding in
the vicinity of burn scars. However, it also seems possible that
the 0.5+ inches of rainfall could fall in as little as 20-30
minutes depending on the degree of convective banding, which could
potentially lead to more significant landslides and flash flooding
in the vicinity of burn scars. The main limiting factor will be a
lack of more anomalous moisture, with higher PWs above the 90th
percentile (for this time of year) generally focused further
south. Nevertheless, LI values falling below -2C (as forecast by
the RAP) are not observed too often in northern California and
this should be considered with respect to rain rates. There are
also a large number of burn scars in the vicinity of the northern
California coastal ranges and Napa Valley, and some of the heavier
rain rates could intersect these locations.

Prior to the arrival of the main wave of heavy rain, scattered
convection will be possible. KBBX radar shows some thunderstorms
already, with a few lightning strikes and cloud flashes noted over
the past couple hours in the I-5 corridor. RAP analysis at 02Z
already has 100-200 j/kg of MUCAPE situated in the northern
Sacramento Valley, with GPS-PW observations on the order of 0.7 to
0.8 inches. This should be sufficient for some isolated 0.5 in/hr
rain rates even before the stronger height falls arrive. Some of
the convection was drifting in the general direction of the Carr
Fire. Although instability does seem to decrease further north in
the valley, the ongoing band of convection does merit monitoring.

Lamers

ATTN...WFO...EKA...HNX...MTR...STO...

ATTN...RFC...CNRFC...

LAT...LON   41132254 41082222 40922193 40432177 39942151
            39632115 39312081 38812055 38372031 37982012
            37642021 37742076 37892141 38052203 37962245
            37912305 38362328 38772385 39532399 40042435
            40392409 40262367 40092350 39962326 39772309
            39412301 39182278 39242263 39612265 39942268
            40232283 40462288 40622272 40922276